Ergothioneine is a sulfur-containing amino acid represented by the following formula (II):

It is a biological compound known to exist in the liver and other organs as well as in the blood of animals, including humans.
Ergothioneine is known to have antioxidative activity. For example, it is said to have an ability to scavenge hydroxyl radicals, an ability to suppress the iron- or copper-dependent generation of hydroxyl radicals from hydrogen peroxide, an ability to suppress copper-dependent oxidation of oxyhemoglobin, and an ability to suppress oxidation of arachidonic acid by myoglobin and hydrogen peroxide. Ergothioneine is also said to show inhibition of elastase activity, inhibition of tyrosinase activity, anti-inflammatory activity, enhancement of cell energy, anti-stress activity, anti-aging activity, anti-wrinkle activity, and suppression of lipid peroxide generation.
Due to its characteristics as a functional biological compound having various physiological activities and as a heat-resistant, water-soluble material, ergothioneine is expected to find applications in functional food products, supplements, cosmetic products, pharmaceutical products, quasi-pharmaceutical products, animal feed, and the like.
Known production methods of ergothioneine include, in addition to extraction from the organs or blood of animals, extraction from mycelium of mushrooms capable of producing ergothioneine (See, Patent Documents 1 and 2 below, the entire disclosure of each of which is incorporated herein by reference). Non-Patent Documents 1 and 2 below (the entire disclosure of each of which is incorporated herein by reference) state that most bacteria lack the ability to produce ergothioneine. These documents also state that certain fungi such as Aspergillus niger and Neurospora crassa are capable of producing ergothioneine whereas Saccharomyces cerevisiae has little ability to produce ergothioneine.
Ergothioneine biosynthesis system have been reported in some fungi capable of producing ergothioneine. For example, Non-Patent Documents 3 and 4 below (the entire disclosure of each of which is incorporated herein by reference) describe the biosynthesis system of ergothioneine in Neurospora crassa, while Non-Patent Document 5 below (the entire disclosure of which is incorporated herein by reference) describes the biosynthesis system of ergothioneine in Schizosaccharomyces pombe, a “fission yeast”.